Internal combustion engines utilize combustion to generate mechanical power. However, the combustion generates heat that can adversely impact the structure of the engine and surrounding components if the heat is not properly dissipated. Accordingly, internal combustion engines are either air-cooled or liquid-cooled. Air-cooled engines used to be prevalent before the technological advancements that have paved the way for liquid-cooling. In liquid-cooled engines, the liquid (e.g., coolant) is expected to operate within some nominal range to ensure stable combustion performance and prevent component failure. If the coolant is too cold, a variety of operational problems can occur such as incomplete combustion, power output losses, and an oil viscosity that is too dense or heavy to provide adequate lubrication in the engine. If the coolant is too hot (e.g., above the nominal range), another set of operational problems can occur such as premature combustion and a degradation of part-tolerances. To maintain the coolant within the nominal temperature range, a thermostat is employed in the cooling system for the engine.
The thermostat regulates the cooler limit of this nominal operation range when put in series with a coolant-to-air heat exchanger (commonly referred to as a radiator). Combustion itself provides the energy to allow the system to attain ideal temperature operation. On-Board-Diagnostics are typically used to identify failure of the thermostat, which may result in an indicator lamp on a dashboard of the vehicle. To date, a challenging application exists in systems which employ large heat exchangers which are not isolated by mechanical thermostats. Urban on-highway bus applications exemplify this scenario, where large capacity coolant-to-cabin air heat exchangers are employed to heat the passenger cabin in cold weather. Because this heat exchange causes coolant temperature to mimic the signature of a system with a failed-open thermostat, current diagnostics are incapable of distinguishing this scenario from a hardware failure. It is desirable to repair a failed thermostat to prevent system damage and degradation in performance.